High-frequency radio amplifying circuits



May 16, 1935.

v w; J. POLYDOROFF HIGH-FREQUENCY RADIO AMPLIFYING CIRCUITS Filed Feb.11, 1936 .wwiw wwmw INV ENTOR MAD/Ml IPO YO0ROFF ATTORNEYyi iii 'cycles.

Patented May is, 1939.

wnaimii- J. l olydoroif, Wllmctte, lll., assignor to JohnsonLaboratories. Inc., Chicago, 111., a

corporation of Illinois Application February 11, 1936, serierne. am

6 Claims (01. 179-171) -The invention relates to radio receivingapparatus and more particularly to permeabilitytuned circuits for use inthe high-frequency range between approximately. and mega-.

The circuit losses at these high fre-.

quencies are of much greater magnitude than at the lower radiofrequencies and additional losses which are unnoticeable at frequenciesbelow about 5 megacycles become predominant. For example, the inputconductance of the usual type of highfrequency thermionic amplifier tubeincreases as the second power of the frequency, and becomes a factor ofreal importance at'the higher radio frequencies. One object of theinvention is the material reduction of the eifect of these losses uponthe selective properties of the variably tuned circuits employed in aradio receiver. Another object of the invention is to provide a novelcircuit'arrangement which is characterized by substantially uniform gainand selectivity throughout this frequency range.

A high-frequency variable inductance device for employment at these highfrequencies preferably has a core of substantially the same length asthe winding. The circuit arrangement of my present invention, whichemploys these variable inductance devices to secure an increasedefficiency at the higher radio frequencies, will be better understood ifthe following description is,

taken in connection with the accompanying drawing, in which:

Fig. 1 is a schematic diagram of a circuit embodying the invention, and

Fig. 2 is a graph showing a comparison of the operating characteristicsof my improved circuit as exemplified by the embodiment of Fig. 1 asagainst those of conventional circuit arrangements.

Fig. 1 shows a circuit suitable for the reception of high-frequencysignals. Antenna circuit l is inductively coupled by means of a smallprimary winding 2 to a selective circuit 3 consisting of winding 4, theinductance of which is adjusted by. movement of ferromagnetic core 5,and capacitor 8. When circuit 3 is tuned to resonance with an incomingsignal, a maximum voltage will be developed across winding 4representing a magnification of the signal voltage of approximatelythree times. mally applied directly to the grid Ia of a thermionic tube'5.

At a frequency at the upper end of the tuning range, for example, 20megacycles, the ferromagnetic core 5 is completely withdrawn from thewinding 4 and the characteristics ofcircuit 3 by The voltage thusobtained is norat its normal efliciency.

itself are determined by the properties of winding 4, capacitor 6 andthe losses which are reflected. through the primary winding 2 from theantenna.

. This circuit as such may readily be designed to produce satisfactoryselective properties. However, if the high-p tential terminal of thiscircuit is connected directly to the grid la of thermionlc tube 1, newlosses are introduced, due to the electronic loading effects of thethermionic tube at high frequencies. As pointed out by Ferris 0 in theProceedings of the Institute of Radio Engineers for January 1936, atpages 82-107, the input conductance of vacuum tubes increases as thesquare of the frequency, andthus seriously affects the performanceobtainable even at moderately high radio frequencies. Thus, while thecircuit itself has desirable characteristics, as indicated by its L/Rratio or its Q", the connection of the circuit to the thermionic tubecauses a reduction ofboth gain and selectivity to approximately onehalfof their original values. The arrangements of the present inventionsubstantially overcome this disadvantage and provide a high degree ofemciency at the higher radio frequencies. In the lower frequency region,however, corresponding to the position of the core inserted all the wayinto the winding 4, the losses due to the electronic load of thethermionic tube are greatly diminished and'the circuit operatessubstantially In accordance with the invention, and as shown in Fig. 1,the grid Ia of tube 1 is connected to a tap 8 on the winding 4. Thusonly a portion of the voltage developed, across thecircuit 3 is ap--plied to the grid of the tube. This arrangement greatly reduces thedamping effect of the tube upon resonant circuit 3, since itsinputconductance is impressed across only a portion of winding 4, andtherefore this arrangement provides greater selectivity than would berealized were 40 the grid lit to be connected directly to the high"-potential terminal of the winding 4-. 'Additionally, since theinductance gradient of the winding 4 is redistributed by the relativemovement of the core 5, the electrical location of the tap 8 is alsovaried. If the winding is uniform and its field is not materiallyafiected by the position of the winding 2, the tap'8 maybeadvantageously located at the electrical center of the winding.Underthese circumstances, when the core is completely withdrawn from thewinding, so that it has substantially no efiect upon the coil, the tapwill also be at the center of inductance of the winding. Hbwever, assoon as the core starts to enter the coil, the inductance of the lowerportion of winding 4 is increased by the presence of the core, so thatthe grid circuit of tube I includes more than one-half of the totalinductance. This is because the core enters that end of the windingwhich is connected to the ground. As the core is advanced further intothe winding, the electrical position of the tap is shifted still furthertowards the high-potential end of the winding, and the value of theportion of the inductance in the grid circuit of tube 1 is still furtherincreased. At the same time, the selectivity tends to be decreased dueto the increased damping effect of tube 1 on the circuit I.

When the core 5 is aboutmidway of its travel, the shift of theelectrical position of the tap 8 is at its maximum. As the core advancespast this position and still further into the winding,

the shift of the electrical position of the tap gradually diminishes,until at the end of the travel, when the core is all the way in, theelectrical position of the tap again corresponds approximately to theoriginal position, and the tap is effectively at the center ofinductance of the winding. Under this condition, the damping effect ofthe input conductance of tube I ,on the selectivity of circuit 3 is aminimum, as explained above with reference to the high-frequencyposition of core 5.

It will be understood that by placing the tap at some point other thanat the electrical center of the winding, the maximum shift in itselectrical position, as the core advances into the winding, may bechanged. Thus by proper choice of the original location of the tap, thedegree of shift may be varied to suit particular conditions, and toovercome or reduce undesired variations in the gain and selectivity ofthe system as it is tuned over a range of frequencies.

In Fig. 2, curve A indicates the gain between the antenna circuit I andthe grid In of tube I in the circuit of Fig. 1, but with grid laconnected to the high-potential terminal of winding 4. Due to' theelectronic load of the tube 1, the amplification is considerably poorerat the upper frequencies than at the lower frequencies, resulting innon-uniform and inemcient performance. The gain due to the voltagedeveloped across the coil 4 depends on the electrical position of thetap 8. At the high-frequency end with the core withdrawn, the gain isreduced; then, as the core moves in, the gain is increased and at thelowest frequency, when the core is fully in, is reduced again. On theother hand, the gain also depends on the damping effect due to the inputconductance of the tube I. At the high-frequency end of the .tuningrange the damping is greatest, but the load reflected in the circuit 3is minimized because of the action of the tap 8. The reduction ofvoltage gain is thus compensated by the reduction'of damping. At thelowest frequency, when damping due to the tube is negligible, anabnormal increase in gain would occur, as shown in curve A; but thisincrease in gain is compensated for by the action of the tap 8, thusproducing substantially uniform gain throughout the entire frequencyrange. Curve B indicates the gain which is realized with the improvedarrangement shownin Fig. 1.

Although the amplification at the higher frequencies remainssubstantially the same as before, in spite of the reduction in thevoltage applied to the grid Ia of the tube 1, because of the diminishedload on tuned circuit 3, the selective properties, indicated in Fig. 2as "selectance, i. e., the band width in kilocycles at half When tuningis accomplished by movement oi a homogeneous ferromagnetic core, adecrea'se in the amplification beyond that to be expected due to thechange in frequency normally occurs in the'middle region of the tuningrange. If the frequency range to be covered is the standard broadcastband from 540 to 1800 kilocycles, this non-uniformity of amplificationis readily overcome by utilizing a core having varying magnetic density,as described in my United States Patent No. 1,982,689. This expedient,however, is dimcult to apply at the higher radio frequencies, since thepermeability of the cores for use at these frequencies is alreadyconsiderably reduced by the necessary finer subdivision of the iron andrelatively thicker film of insulation between adjacent particles.

For these higher radio frequencies, my present invention provides asimple and advantageous arrangement for securing substantially uniformamplification and substantially constant selectance with a homogeneouscore. At an intermediate position of the core 5, corresponding to afrequency near the center of the range, the redistribution of theinductance gradient of the winding 4 causes the tap 8 to electricallyshift toward the high-potential terminal of winding 4. Thus a highervoltage is applied to grid Ia and the amplification of the circuit isincreased at the middle frequencies. When the core is all the way in thewinding 4, however, the tap 8 is electrically shifted back to itsoriginal position near the electrical center of the winding 4, and theamplification is, therefore, about onehalf what it would be were thegrid 10 of tube I to be connected to the high-potential terminal ofwinding 4. With this arrangement, not only is the gain substantiallyuniform throughout the tuning range of the system but also the selectiveproperties are greatly improved, as indicated by curve C, due to thereduction in loading of the tuned circuit 3 by the tube 1.

The thermionic tube 1 may be connected to a second tuned circuit 9, anda tap III on the winding ll may be employed to apply a voltage to thegrid of a second thermionic tube l2. The second circuit will havesubstantially the same improved performance over a range of frequenciesas does the first circuit, and the cores 5 and I3 may be ganged forsimultaneous movement, to simultaneously .and synchronously tune the twocircuits. It will be understood that tube I or tube l2 may functionotherwise than as an amplifier without departing from the scope of theinvention.

The circuit of Fig. 1 may be employed in the preselector system of asuperheterodyne radio receiver, or any desired number of stages of thetype shown may be operated in cascade in a receiver of the tunedradio-frequency type. In either case, by properly locating the tappedgrid connection on the windings, the receiver may be designed to havesubstantially uniform amplification and considerably more. nearlyconstant selectivity over a range of high radio frequencies.

Having thus described my invention, what I claim is:

1. A high-frequency system including a resonant circuit and .a vacuumtube having a. grid, said resonant circuit including a fixed capacitorand a variable inductor having a winding and a high-frequencyferromagnetic core movable relatively thereto to vary the inductance ofsaid winding and thereby tune said circuit over a range of frequencies,and means whereby a variable portion of the resonant voltage developedacross said winding is applied to said grid, said portion depending uponthe position of said core relative to said winding.

2. A high-frequency system including a resonant circuit and a vacuumtube having a grid,

said resonant circuit including a fixed capacitor .said resonant circuitincluding a fixed capacitor and a variable inductor having a winding anda high-frequency ferromagnetic core movable relatively thereto to varythe inductance of said winding and thereby tune said circuit over arange of frequencies, and means including a connection from said grid toa point on said winding whereby a variable portion of the resonantvoltage developed across said winding is applied to said grid, saidportion depending upon the position of said core relative to saidwinding.

4. A high-frequency system including an input circuit, a resonantcircuit inductively coupled to said input circuit, and a vacuum tubehaving a grid, said resonant circuit including a fixed capacitor and avariable inductor having a winding, a tap on said winding and aconnection from said tap to said grid, and a high-frequencyferromagnetic core movable relatively to said winding for simultaneouslytuning said circuit over a range of frequencies and altering theelectrical position of said tap to secure substantially uniform gainthroughout said range.

5. A high-frequency system including an input circuit, a resonantcircuit inductively coupled to said input circuit, and a vacuum tubehaving a grid, said resonant circuit including a fixed capwcitor and avariable inductor having a winding, a tap on said winding and aconnection from said tap to said grid, and a high-frequencyferromagnetic core movable relatively to said winding for simultaneouslytuning said circuit over a range of frequencies and altering theelectrical position of said tap to secure substantially constantselectance throughout said range.

6. A system for operation in a range of high frequencies including aresonant circuit and a vacuum tube having a grid and high grid-circuitlosses in said range, said resonant circuit including a fixed capacitorand a variable inductor having a winding, a tap on said winding and aconnection from said tap to said grid to materially decrease the eifectof said losses upon said resonant circuit, and a high-frequencyferromagnetic core movable relatively tosaid winding for simultaneouslytuning said circuit over said range and altering the electrical positionof said tap to secure substantially uniform gain and substantiallyconstant selectance throughout said range.

WLADIMJR J. POLYDOROFF.

